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SPERMATOPHYTA 415 
In the cycads the prothallus, or female gametophyte, is 
retained permanently within the sporophyte, and even the young 
sporophyte begins its development under the protection of the 
previous sporophytic generation. Thus the gametophyte, which 

Fia. 468. Pollen grains and pollen tubes of Cycas 
Upper left, pollen grain of C. rumphii before division of nucleus (x 1080). 
Lower left, pollen grain of C. rwmphii after the formation of male prothallus 
(x 1080) ; above is the large tube cell, below this the generative cell, below the 
generative cell the prothallial cell. Center, pollen tube of C. rumphii before 
division of cell (x 87): t, tube nucleus ; 6, body cell; s, stalk cell ; p, prothal- 
lial cell. Right, pollen tube of C. circinalis with two spermatozoids. (Last 
figure redrawn after Miyake) 
in the liverworts is the dominant phase and in the ferns an 
independent plant, is reduced in the cycads to a condition in 
which it is parasitic on the sporophyte. On the other hand, the 
sporophyte is much more highly specialized in the cycads than 
in any of the pteridophytes. 
Relationship. The cycads are descended from an extinct order 
of plants, the Cycadofilicales, which in turn were derived from 
